Electron lens for multiplier phototubes with very low spherical aberration



R. G. STOUDENHEIMER ET AL ELECTRON LENS FOR MULTIPLIER PHOTOTUBES Dec.20, 1955 WITH VERY LOW SPHERICAL ABERRATION Filed April 26, 1951 bez'mera mm N d u A wfl E W M n Ra] BY 2 71 United States Patent O ELEC RDN LENS F OF. MULTIPLIER PHOTOTUBES /3TH VERY LGW SPHERECAL ABERRATION RichardG. Stoudenheimer and Ralph W. Engstrom, Laneasier, Pa, assignors toRadio Corporation of America, a corporation of Delaware AppiicafionApril 26, 1951, Serial No. 222,998

7 Claims. (Cl. 313-95) This invention relates to a phototube, and morespecifically to a large area photoemissive cathode for use inphototubes, such as high vacuum photomultiplier tubes.

Photomultiplier tubes having large area photocathode surfaces whereinthe photosurface is closely coupled with a phosphor layer, have beenused as scintillation counters. The operation of the tube is such thatradiations from radioactive materials are caused to fall upon a phosphorsurface and activate it to luminescence, the light of which in turncauses photoemission from the photocathode of the tube. in suchapplications, it is necessary that all electrons emitted by thephotocathode be directed into the multiplier section of the tube tocontribute to the output signal. To achieve this, it is necessary forelectrons emitted from any point on the photocathode surface to bedirected or focussed into a restricted region or point. Normally, thelarger the photocathode of such tubes, the more pronounced are theaberrations of the focussing system which increase the size of thefocussed spot. Thus, electrons, particularly those originating from theedges of the photocathode, are scattered and not collected by themultiplier section. This results in a loss of signal.

It is thus an object of the invention to provide a phototube having alarge area photocathode and wherein the electrons from the photocathodeare directed into a restricted region.

it is another object of the invention to provide a pho totube having alarge area photocathode in which the focusing of the photoemission fromthe photocathode is of low aberration.

It is a further object of the invention to provide a photomultipliertube having a large area photocathode and adjacent tube portionsdesigned to direct the photoemission into a restricted region.

The foregoing and related objects are achieved in accordance with theinvention by providing-a tubular The tubular envelope is' envelope forsaid phototube. closed by an end wall having, at its center, asubstantially concave spherical surface and, at its peripheral portions,annular concave surfaces of increasing radii with respect to thedistance from the center. Also the peripheral surfaces of the face platemay be formed with a negative radius of curvature forming a convex innersurface.

Figure 1 is a sectional view of a photomultiplier tube large-area lightsources, such as scintillation counters for' the detection andmeasurement of nuclear radiation.'

The tube comprises essentially ,'a glass envelope 1 0,

closed at one end with a transverse wall section 12, upon which isformed a transparent photocathode surface 14.

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In one tube of this type, end-wall portion 12 has a diameter ofapproximately two inches. The exposed portion of the photocathode film14 is approximately 1 /2 inches. This provides a useful, large,substantially flat cathode area which permits good optical couplingbetween the photocathode and the surface of a phosphor screen, used inscintillation counters, for example.

Spaced from the photocathode and along the tube axis is an acceleratingelectrode or portion 16 formed as a disc and having an aperture 17 atits center. A metallic wall coating 18 is formed on the inner surface ofthe tube envelope and extends from the photocathode film 14 axially downthe tube to a portion below the accelerating electrode portion 16. Wallcoating 18 provides electrical contact between the photocathode 14 and alead 20, connecting the photocathode and the metallic film 18 to asource of ground potential, as shown.

As indicated in Figure 1, a potential difference of 150 volts ismaintained as an optimum value between accelerating electrode 16 and thephotocathode 14. Photoelectrons emitted from cathode 14 are thusaccelerated toward electrode 16. Wall coating 18 aids in directing thephotoelectron toward the opening 17.

Photoelectrons passing through opening 17 are collected by an electronmultiplier section 22, which consists of a plurality of dynodeelectrodes (shown in dotted lines) enclosed in a cylindrical metalshield 24. The photoelectrons will impinge upon an anode collectorelectrode which is also a first dynode electrode 26 and will initiatesecondary emission therefrom having a ratio greater than unity. Thissecondary emission is accelerated and directed by a fixed electrostaticfield along curved paths to successive dynodes. Each dynode provides anamplification of the electrons striking it to form an ever-increasingstream of electrons until those emitted by the last dynode are collectedby a collector or anode electrode 28. The current collected by collectorelectrode 28 constitutes the current utilized in the output circuit ofthe tube. This type of electron multiplier is fully described in U. S.Patent 2,285,126, of Rajchman et al. The specified details of themultiplier do not com stitute my invention.

Opening 17 into the multiplier section 22 is covered by a mesh grid 30.This grid is connected electrically to the anode collector or firstdynode 26 and tends to prevent secondary electrons, from dynode 26, frompassing back toward the photocathode 14. The first dynode electrodeportion 26 is fixed to the accelerating disc portion 16 and is, thus,tied electrically to it. In normal tube operation, apotential differenceof about 75 volts is maintained between each of the succeeding dynodestages. I p

Commercial tubes, of the type described, have been made with aphotosurface formed by putting down on the end wall 12 asemi-transparent film of manganese which is oxidized and then depositingthereon a thin film or layer of antimony which is sensitized bycondensing a deposit of cesium metalthereon. This type of photoemissivesurface is fully described in the copending application of J. J.Polkosky, Serial No. 219,997 filed April 9, 1951, now U. S. Patent2,676,282 which issued April 20, 1954. Such a photocathode surface has aspectral response, which may be varied over a range between 3000 A. to6400 A. The response of the material is peaked at around 4800 Angstroms.

In the operation of tubes of the type shown in Figure 1, it is desirablethat photoelectrons from all portions of the photocathode film 14 maycontribute to the signal. It is thus necessary that the photoelectronsoriginating at all points of the photocathode be directed or guided intothe restricted region of aperture 17, so that they can pass into themultiplier section and strike the dynode electrode 26. To focus ordirect an electron emission from a large surface to a common point, atheoretically ideal cathode surface would be that having a sphericalconfiguration. Thus, apositive voltage at the center of curvature of thesurface would guide the electronemission into a small region at thecenter of curvature.

However, in tubes of the type described, the photo 2, an additionaleffect of a negative wall coating 18a.

upon electrons leaving the photocathode 14a is to shorten the focaldistance from the photocathode at which the electron paths cross. Thiseffect is greatest in the region of the photocathode nearest the wall10a. Electrons leaving the edge of the photocathode film 14a Will crossat points closer to the photocathode than will the paths of electronsleaving the center portions of the film. The paths of the electrons asshown in Figure 2 disclose considerable aberration in the electronoptics composed of the anode 16a, the photocathode 14a, and wall coating181:. Thus a tube made with a spherical end wall does not providecomplete collection of photoelectrons from all points of thephotocathode. Photoemission from the edge portions of the photocathodetend to miss the opening 17 into the multiplier section and strikeportions of accelerating electrode 16, instead. These electrons do notcontribute to the output signal of the tube.

In accordance with the invention, all portions of the photocathode 14(Fig. l) are provided substantially with a common focal length byproperly forming the shape of the wall portion 12 of the tube. As shownin Figure 3, the end wall 12 is formed with a central portion whoseinner surface facing accelerating electrode 16 is concave andsubstantially spherical with a center of curvature 40 on the axis 42 oftubular portion 10 of the envelope. Between this center portion and theedge of the wall 12, the intermediate annular portion is alsosubstantially spherical with a larger radis of curvature, R2 forexample. The annular region at the periphery of the end wall 12 andcontiguous to the tubular wall 10 may be flat in a plane perpendicularto axis 42, or may have a negative radius of curvature, Rs so that theperipheral annular region has a convex inner surface as seen from anode16, and as shown in Figure 3.

The effect of a photosurface, of the type described for Figure 3, is onein which the electrons emitted from photocathode 14 follow paths towardaccelerating electrode 16, which will converge to a common restrictedregion within or closely adjacent the opening 17 of the multipliersection and as indicated by the convergence of the dotted lines ofFigure 4. Such a photosurface provides the minimum aberration effects ofthe focusing fields between cathode 14 and accelerating electrode 16.The proper curvature of the inner surface of end' wall 12 of Figure 3can be determined on a rubber membrane mode for the desired electrodestructures used. In one commercially operated tube of the typedescribed, shown in Figure 3, the radius of curvature R1 of the centersection is 3 inches. The radius of curvature R3 of the peripheral regionis /2 an inch and the center of curvature 44 of this surface issubstantially of an inch from the axis 42 of the tube. The diameter ofthe tubular portion 10 is two inches.

Figure discloses a modification of theinvention which the centralportion of the end wall 12 of the tube is formed with aconcave sphericalinner surface having a radis R1 and a center of curvature 46 on the axis48 of the tubular envelope portion 50. However, the annular peripheralregions extending between the center portion of radius R1 to the'edge ofthe end plate 12 have also substantially spherical surfaces ofdecreasing curvature with distance from axis 46. That is the radius ofcurvature of the end plate 47 increases with respect to the distancefrom the axis 48 of the tube envelope, as indicated by R; and R5, forexample. A photocathode film, deposited on the end wall 47 of Figure 5,will also provide an electron emission which will be directed or focusedto a small confined region on the axis of tube. Such a photosurface alsowill provide less aberration effects than that provided by a sphericalsurface of single radius as shown in Figure 2.

From the foregoing, it will be apparent that the invention provides animproved phototube having a large photocathode surface characterized byits improved electron focusing and collecting characteristics asdisclosed.

We claim:

1. A phototube comprising, a photoemissive cathode, and an anodeelectrode spaced from said cathode, said cathode having a concavesurface facing said anode, said cathode surface having at its centerportion a substantially spherical curvature of one radius and anadjacent annular spherical portion having a curvature of larger radiusthan said one radius.

2. A photoelectron discharge device comprising, an envelope, atphotoemissive film on the inner surface of a portion of said envelope,an anode electrode mounted within said envelope and spaced from saidphotoemissive film, said envelope portion having a curved surface ofincreasing radius of curvature relative to the distance of the surfacefrom its center.

3. A phototube comprising, a tubular envelope, an end wall closing oneend of said envelope, a photocathode film formed on the inner surface ofsaid end wall, an anode electrode spaced within said envelope from saidphotocathode, said cathode surface having at its center portion asubstantially spherical curvature of one radius and an adjacent annularspherical portion having a curvature of larger radius than said oneradius.

4. A phototube comprising, an envelope having a tubular portion, atransparent end wall closing one end of said tubular envelope portion, aphotoemissive film on the inner surface of said end wall, and an anodeelectrode on the axis of said tubular envelope portion within saidenvelope and spaced from said photoemissive film, said inner surface ofsaid end wall at its center portion being concave outwardly, theportions of said end wall respectively having curvatures with increasingradii relative to the distance from the center of said end wall.

5. A phototube comprising, an envelope having a tubular portion, atransparent end wall closing one end of said tubular envelope portion, aphotoemissive film on the inncrsurface of end wall, and an anodeelectrode on the axis of said tubular envelope portion within saidenvelope and spaced from said photoemissive film, the center portion ofsaid inner surface of said end wall, facing said anode electrode being aconcave substantially spherical surface with a center of curvature onsaid axis of said tubular envelope portion, portions of said end wallbetween the wall of said tubular envelope portion and said center endwall portion having annular spherical surfaces concave relative to saidanode electrode and of decreasing curvature relative to the distancefrom the center of said face plate.

6. A phototube comprising, an envelope having a tubular portion, atransparent end wall closing one end of said tubular envelope portion, aphotoemissive film on the inner surface of said end wall, and an anodeelectrode on the axis of said tubular envelope portion within saidenvelope and spaced from. said photoemissive film, the center portionof-said inner surface of said end wall facing said anode electrode beinga concave substantially spherical surface with a center of curvature onsaid axis of said tubular envelope portion, and peripherical portions ofsaid end wall contiguous the wall of said tubular envelope portionhaving an annular surface of convex curvature relative to said anodeelectrode, said photoelectric film extending over said convex annularwall surface.

7. A phototube comprising, an envelope having a tubular portion, atransparent end wall closing one end of said tubular envelope portion, aphotoemissive film on the inner surface of said end wall, and an anodeelectrode on the axis of said tubular envelope portion within saidenvelope and spaced from said photoemissive film, the inner surface ofsaid end wall facing said anode electrode being concave and having anincreasing radius of curvature relative to the distance of the surfacefrom its center, and the peripheral portions of said end wall whichcontact the wall of said tubular envelope portion having an annularspherical surface of convex curvature relative to said anode electrode.

References Cited in the file of this patent UNITED STATES PATENTS1,460,501 Ritter July 3, 1923 1,929,228 Wilhelm Oct. 3, 1933 2,054,884Schlesinger Sept. 22, 1936 2,179,083 Bruche et al. Nov. 7, 19392,231,698 Zworyking et al. Feb. 11, 1941 2,238,634 Flechsig Apr. 15,1941 2,244,720 Massa et a1 June 10, 1941

